I just checked the North American Novaworld and the top servers were 92/150, 64/64, 54/64, 50/64 and they go down like that. There is usually similar servers for the expansion pack but that tends to have a few more bigger games going. I usualy play 64 player games, but the 150 server is usually pretty full for bigger games.
Depending on the map, it can be a slow-paced game, if you have to take a long boat ride or whatever. So I stick to the maps that aren't ridiculously large. But it's a ton of fun always, but I wish they would release some more maps, especially CQB ones.